This invention relates generally to industrial process control instruments, and more particularly to a front panel for such instruments constituted by a single gas-discharqe display panel which integrates several indicators and control elements.
An electronic controller is a component in a process control loop that is subject to disturbances, the controller acting in conjunction with other devices to maintain a process variable at a desired value. The factor controlled may be flow rate, pressure, viscosity, liquid level, or any other process variable. In operation, the electronic controller receives, in terms of corresponding input signals, both the process variable and a set point, and it compares these electrical values to produce an output signal that reflects the deviation of the process variable from the set point. This output signal, when applied to a final control element, will directly or indirectly govern the process variable.
Thus one input signal to a controller may be derived from a flowmeter whose reading is converted into a corresponding electrical value, and the output signal may be impressed on a flow-regulating valve which is caused to assume an intermediate position between open and closed at which the flow rate conforms to the set point. The set point generator may be an internal component of the controller or a remotely-controlled device.
Variations in controller action are obtained by adjustment of parameters associated with the control modes and are available in several combinations. These modes of control action which are combined to adjust the controller output signal are known as proportional, reset and derivative.
Proportional action produces an output signal proportional to the deviation of the controlled process variable from the set point. The amount of deviation in terms of percentage required to move the final control element through the full range is known as the proportional band. Automatic reset action, also known as integral action, produces a corrective signal proportional to the length of time the controlled variable has been away from the set point, while derivative action, also known as rate action, produces a corrective signal proportional to the rate at which the controlled variable is changing. Manual reset action is an operator-actuated potentiometer controlled to produce a corrective signal directly proportional to the magnitude of the adjustment.
The concern of the present invention is with the front panel of an electronic controller, and for purposes of background we shall consider the front panel included in the Fischer & Porter (Warminster, Pa.) "Total View Electronic Controller" as described in their Instruction Bulletin for the Series 53EG4000 published in 1979; for this instrument is typical of commercially-available electronic controllers.
The front face of the Total View Electronic Controller displays process, set point and final control element information and it also contains those operator controls necessary for the manual and automatic operation of the instrument.
Thus the front panel includes a process variable and set point indicator consisting of a common direct reading vertical scale graduating from 0-100% and two meter-movement pointers; one in red to indicate process, and the other in black to indicate set point along the same scale. Also mounted on the front panel below this indicator is an output meter having a 0-100% horizontal scale whose pointer indicates the direct current output of the meter.
To permit selection of automatic or manual operation, two push-buttons are provided on the front panel for this purpose. To effect operation of the final control valve in either direction when the manual transfer push-button is depressed, two manual drive push-buttons are provided. Also on the front panel is a set point thumb wheel which turns a set point potentiometer to provide the necessary set point voltage.
Thus the front panel of the typical electronic controller consists of several discrete indicators, switches and other control elements in a fairly complicated and costly assembly which leaves much to be desired in terms of human engineering and modern industrial design.
When solid-state indicators such as LED, LCD and gas-discharge displays are commercially available, and indicators of these types can be used to replace the electromechanical indicators installed on the front panel of conventional controllers, the resultant display arrangement would still be comprised of discrete components and have the drawbacks thereof.